The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for current matching. Merely by way of example, the invention has been applied to current matching of LED channels. But it would be recognized that the invention has a much broader range of applicability.
Liquid crystal displays (LCDs) have been widely used in various electronics products. A LCD panel usually does not have a self-illuminating property. A backlighting source often needs to be used to illuminate the LCD panel from the back of the LCD panel. Each pixel of the LCD panel often filters the light from the backlighting source differently to produce images. Light emitting diodes (LEDs) have been used in backlighting for LCDs. When multiple channels of LEDs are implemented for backlighting, a reference current can be provided to generate channel currents for driving LEDs, and the error of the channel currents is usually no more than 2% in order to evenly backlight a LCD screen.
FIG. 1 is a simplified conventional diagram showing a system for driving multiple channels of LEDs with a reference current. LED channels include channels 1021, . . . , 102n, where n is no less than 1, and each of these LED channels has one or more LEDs connected in series. A dynamic head room control unit 104 generates a control signal 106 which is received by a switching-mode power system 108. In response to the control signal 106, the switching-mode power system 108 generates a voltage signal 110 to one end of each of the LED channels 1021, . . . , 102n. Voltages at the other end of each of the LED channels 1021, . . . , 102n are provided to the dynamic head room control unit 104.
In addition, a current balancing structure 112 includes a channel reference generator 116, and channel drivers 1181, . . . , 118n. The channel reference generator 116 receives a reference current 120, and generates channel driving currents 1221, . . . , 122n. The channel driving currents 1221, . . . , 122n are received by the channel drivers 1181, . . . , 118n, respectively. Then the channel drivers 1181, . . . , 118n provide channel currents 1241, . . . , 124n to the LED channels 1021, . . . , 102n, respectively. The channel drivers 1181, . . . , 118n can have similar structures and perform similar operations.
FIG. 2 is a simplified conventional diagram showing certain components of one of the channel drivers 1181, . . . , 118n. As shown, the channel driver 200 (e.g., the channel driver 1181) includes an operational amplifier 202, two resistors 204 and 206, and a transistor 208. For example, the transistor 208 is an N-P-N bipolar junction transistor (BJT). In another example, the operational amplifier 202 includes one or more N-P-N BJTs.
The channel driver 200 receives a current signal 210 (e.g., the channel driving current 1221) which flows through the resistor 204 (e.g., the resistor 128). The operational amplifier 202 receives a voltage signal 212 at an input terminal 216, and in response generates an amplified signal 218. The amplified signal 218 is received by the transistor 208 which is also coupled to another input terminal 220 of the operational amplifier 202. As a result, the transistor 208 generates a channel current 222 (e.g., the channel current 1241) which flows through a LED channel (e.g., the LED channel 1021), the transistor 208 (e.g., the transistor 132), and the resistor 206 (e.g., the resistor 130).
As shown in FIG. 2, the channel current 222 can be determined based on the following equation:
                              I          out                =                                                            K                ×                R                ×                                  I                  ch                                            -                              V                os                                      R                    =                                    (                              1                -                                                      V                    os                                                                              I                      ch                                        ×                    KR                                                              )                        ×            K            ×                          I              ch                                                                    (                      Equation            ⁢                                                  ⁢            1                    )                ⁢                                      
where Iout represents the channel current 222, K×R represents the resistance of the resistor 204, and Ich represents the current signal 210. Additionally, Vos represents an input offset of the operational amplifier 202, R represents the resistance of the resistor 206, K×R×Ich represents the voltage signal 212, and Vos/R represents an error term.
Referring to FIGS. 1 and 2, various non-ideal factors can adversely affect the matching of channel currents 1241, . . . , 124n. These non-ideal factors include resistance mismatching, mismatching of channel driving currents, and the existence of input offset (e.g., Vos). Further, these non-ideal factors can change significantly with different manufacturing technologies. Thus, it is often difficult to match channel currents of different LED channels (e.g., the channel currents including 1241, . . . , 124n. Through proper device size and good layout matching, resistance magnitudes of different channels can be matched (e.g., within an error of about 0.1%), and driving currents of different channels can also be matched (e.g., within an error of about 1%). Therefore, the existence of input offset (e.g., Vos) can be a major factor in channel current mismatching.
For example, the voltage signal 212 is only about 100 mV or less. In contrast, the input offset (e.g., Vos) can be as large as 10 mV for the CMOS technology. Thus, it can be difficult to reduce the mismatching error of the channel currents of the different LED channels (e.g., the channel currents 1241, . . . , 124n) to less than or equal to 2%. In order to improve matching of the channel currents, one option is to use P-N-P BJTs in the operational amplifier 202. But, for such P-N-P BJTs, a lateral structure is often required, which can increase the manufacturing difficulty. Also, additional circuits may also be needed because the current gain (e.g., β) of a P-N-P BJT usually is lower than an N-P-N BJT.
Hence it is highly desirable to improve techniques of current matching of LED channels.